1. Field of the Invention
This invention relates to a two-wire type amplifier suitable for transmitting a combination of an electric power and a signal current through the same line.
2. Description of the Prior Art
The two-wire type amplifier is used for transmitting a detected signal in the form of current converted by a remote receiving unit. The detected signal is produced through a detection by a detector of various quantities measured in industrial processes. The two-wire type amplifier uses a single two conductor cable for the transmission of electric power and for signal transmissions, and does not necessitate separate cables exclusively used for the respective electric power and signal transmissions. Thus, the number of cables necessary is greatly reduced and, hence, the two-wire type amplifier is now prevailingly employed for measurements in industrial processes.
With improvements of the performance of differential amplifiers resulting from a remarkable development of semiconductor techniques, the two-wire type amplifier may be constructed by using integrated circuit differential amplifiers even if the detector produces a relatively large output.
In general, a single integrated circuit differential amplifier has been used as the differential amplifier used in the two-wire type amplifier.
There is an example of the signal detecting circuit using a plurality of semiconductor strain gauges arranged in a bridge. In this example, the mechanical distortions of the semiconductor strain gauges produced by an external force applied to them are transformed into the resistance changes of the respective gauges. These resistance changes are derived from the bridge circuit in the form of the corresponding voltage changes which in turn is inputted to the two-wire type amplifier.
Such a two-wire type amplifier exhibits a steady operation when it is subjected to a small variation of ambient temperature. When it is subjected to a large variation of ambient temperature, however, the signal current produced is different with respect to the same distortion. This is a fatal disadvantage.
The reason for this is that the temperature coefficients of semiconductor strain gauges and resistors both constituting a bridge circuit are different from those of the feedback resistor for feeding back a part of the output signal of the differential amplifier to the bridge circuit connected at the input side of the differential amplifier.
Thus, when the temperature coefficients of the feedback resistors are different from those of the strain gauges, even if those of the strain gauges are equal, the voltage drop across feedback resistor exhibits a different value with respect to the same external pressure, if the ambient temperature changes. As a result, the signal current from the two-wire type amplifier also changes, which in turn gives errors.
Further, in the conventional amplifier, the voltage fed from the measuring circuit to the differential amplifier must be within the operating voltage range (generally, a positive and negative voltages) of the differential amplifier. For this, a high resistance resistor must be connected to the input side of the differential amplifier. The high resistance resistors must be those of a high precesion type, with a very small temperature coefficient and a little dispersion.
Thus, the precision of the conventional amplifier using a differential amplifier largely depends particularly on the temperature coefficient of the high resistance resistor. Various compensating circuits for compensating for the temperature coefficient error have been devised, but they are complicated in circuit construction and insufficient in their compensating performance.
Another problem of the conventional amplifier resides in that there is a dispersion in the temperature coefficients of the differential amplifiers per se. More precisely, an off-set voltage appears at each of two input terminals of the differential amplifier when ambient temperature changes, although these off-set voltage must essentially be zero. The off-set voltage is adversely different each differential amplifier.
This means that, in manufacturing the two-wire type amplifier, the differential amplifier with its off-set voltage within a predetermined value range must be selected from a large sample of amplifiers, in order to secure a predetermined precision of the amplifier. This selection process is very time consuming and thus the selected amplifiers are very expensive.